Washington -LRB- CNN -RRB- -- The Fukushima Daiichi disaster is focusing attention on a problem that has bedeviled Washington policymakers since the dawn of the nuclear age -- what to do with used nuclear fuel .

Currently , spent fuel -- depleted to the extent it can no longer effectively sustain a chain reaction -- is stored in large pools of water , allowing the fuel to slowly cool and preventing the release of radiation .

But events in Japan , where two of the six spent fuel pools at the Fukushima Daiichi facility were compromised , have raised questions about practices at the nation 's 104 nuclear reactors , which rely on a combination of pools and dry casks to store used fuel .

`` I truly believe we must re-think how we manage spent fuel , '' Sen. Dianne Feinstein , D-California , said at a Senate appropriations subcommittee hearing Wednesday .

In California , Feinstein said , fuel removed from reactors in 1984 is still held in spent-fuel pools , well beyond the minimum five to seven years required by federal regulators . `` It 's hard to understand why the Nuclear Regulatory Commission has not mandated a more rapid transfer of spent fuel to dry casks , '' Feinstein said .

Currently , there is no maximum time fuel can remain in spent fuel pools , the NRC said Wednesday . As a result , critics say , nuclear plants have made fuel pools the de facto method of storing fuel , crowding pools with dangerous levels of fuel , industry critics say .

As of January 2010 , an estimated 63,000 metric tons of spent fuel was in storage at U.S. power plants or storage facilities , according to the Nuclear Regulatory Commission .

`` For the history of our nuclear power program , I would say , the storage of spent fuel ... has been an afterthought , '' Ernest Moniz , a physics professor at the Massachusetts Institute of Technology , testified . `` I believe we should really start thinking hard about consolidated storage , presumably in federal reservations , to solve a host of problems . ''

The NRC and industry critics differed on whether spent fuel pools are safe .

`` Spent fuel pools are considered ` safety significant ' systems , so they meet a lot of the same standards that the reactor itself would have to meet , '' said Greg Jaczko , chairman of the NRC . `` For example , the spent fuel pools themselves are required to withstand the natural phenomena like earthquakes and tsunamis that could impact the reactor itself . ''

David Lochbaum , a nuclear physicist with the Union of Concerned Scientists , disagreed .

`` At many sites there is nearly 10 times as much irradiated fuel in spent fuel pools as in the reactor core , '' he said . `` The spent fuel pools are not housed in robust concrete containment structures designed to protect the public from the radioactivity they contain . Instead the pools are often housed in buildings with sheet metal siding like that in a Sears storage shed , '' Lochbaum said .

`` I have nothing against the quality of Sears storage sheds but they are not suitable to nuclear waste storage , '' he said .

A nuclear industry representative said the `` lack of a national strategy '' on waste storage is exacerbating the problem , since it does not know whether to place spent fuel in permanent , on-site containers , or containers suitable for transport .

`` We want to limit the number of times we have to handle used fuel . We want to be able to take it out of the pool once , put it in a cask ... Not all casks are designed for transportation for example , '' said William Levis , a power company president speaking for the Nuclear Energy Institute .

Jaczko said spent fuel pools do n't endanger the public . `` We do n't have a maximum time -LRB- fuel can stay in the pools -RRB- , '' he said . `` But we do analyze the fuel . -LRB- Fuel -RRB- goes through a very rigorous analysis to ensure that -LRB- it can be added to the pool -RRB- safely and securely . ''

A high-ranking energy department official , meanwhile , said a commission studying the issue of spent fuel will issue an interim report by July 29 . The commission was formed after the Obama administration killed a plan to store nuclear waste at Yucca Mountain in Nevada .

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Events in Japan have raised questions about spent-fuel storage in the United States

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The U.S. has 104 nuclear reactors , which rely on pools of water and dry casks for storage

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An estimate by the NRC said some 63,000 tons of spent fuel was stored in the U.S.

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The NRC and industry critics disagree on whether spent fuel pools are safe